How to calculate a logistic sigmoid function in Python?

python sigmoid
Richard Knop picture Richard Knop · Oct 21, 2010 · Viewed 296.9k times · Source

This is a logistic sigmoid function:

enter image description here

I know x. How can I calculate F(x) in Python now?

Let's say x = 0.458.

F(x) = ?

Answer

unwind picture unwind · Oct 21, 2010

This should do it:

import math

def sigmoid(x):
  return 1 / (1 + math.exp(-x))

And now you can test it by calling:

>>> sigmoid(0.458)
0.61253961344091512

Update: Note that the above was mainly intended as a straight one-to-one translation of the given expression into Python code. It is not tested or known to be a numerically sound implementation. If you know you need a very robust implementation, I'm sure there are others where people have actually given this problem some thought.

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